Project Title:Protected Lettuce: an Integrated Approach to Aphid and Caterpillar Control

Project Title:Protected Lettuce: an Integrated Approach to Aphid and Caterpillar Control

Project title:Protected lettuce: an integrated approach to aphid and caterpillar control

Report:Final Report, January 2001

Project number:PC 132

Project leader:Professor G. M. Tatchell,

Head of Entomological Sciences Department,

Horticulture Research International

Wellesbourne, Warwickshire, CV35 9EF

Tel: 01789 470440Fax: 01789 470552


Project manager:R. J. Jacobson.

Stockbridge Technology Centre Ltd (STC) (Formerly HRI*)

Stockbridge House, Cawood, North Yorkshire, YO8 3TZ

Tel: 01757 268 275Fax: 01757 268 996


Key workers:STC

Dr P. CroftMiss N. Mason

Ms K RussellMr J. Hadlow

HRI, Wellesbourne

Ms L. Vice

Location:STC, HRI Wellesbourne, growers nurseries

Project co-ordinators:Graham Ward and Derek Hargreaves

Date commenced:1 June 1997

Expected completion:31 December 2001

Key words:Protected lettuce, aphids, caterpillars, integrated pest management, IPM, biological control, currant lettuce aphid, Nasonovia ribisnigri, peach potato aphid, Myzus persicae, glasshouse potato aphid, Aulocorthum solani, potato aphid, Macrosiphum euphorbiae

Whilst reports issued under the auspices of the HDC are prepared from the best available information,

neither the authors or the HDC can accept any responsibility for inaccuracy or liability for loss,

damage or injury from the application of any concept or procedure discussed.

The contents of this publication are strictly private to HDC members. No part of this

publication may be copied or reproduced in any form or by any means without

prior written permission of the Horticultural Development Council.


I declare that this work was done under my supervision according to the procedures described herein and that this report represents a true and accurate record of the results obtained.

Signature ......

R. J. Jacobson

Stockbridge Technology Centre (Formerly HRI, Stockbridge House)

Cawood, Selby, North Yorkshire, YO8 3TZ

Tel. 01757 268275, Fax 01757 268996

Date ......

Report authorised by ......

G. M. Tatchell,

Project Leader

Head of Entomological Sciences Department,

Horticulture Research International


Warwickshire, CV35 9EF

Tel: 01789 470440Fax: 01789 470552


Date ......


Practical Section for Growers

Commercial benefits of the project 1

Background and objectives 1

Summary of the work and main conclusions 1

Action and information points for industry 8

Anticipated practical and financial benefits from the study 9

Appendix 1 – sampling method11

Science section

  1. Introduction12
  1. Monitoring aphid invasion in lettuce crops17
  1. Monitoring moth activity18
  2. Exclusion of moths and aphids20
  3. Control of aphids with entomopathogenic fungi 32
  4. Technology Transfer 36
  5. References36
  6. Acknowledgements 36
  7. Figures37

©2001 Horticultural Development Council


Commercial benefits of the project

The project has laid the foundation for a new supervised pest control strategy that could ultimately reduce insecticide usage by over 75% compared to routine spray programmes in protected lettuce. In the longer term, this will help to satisfy customer demands for a more general reduction in pesticide residues and should therefore improve the competitiveness of the UK product. However, the strategy will increase production costs by an estimated £1,737 per 1000m2 per annum, which will not be financially viable for growers unless food retailers and consumers recognise that the products have added value and therefore warrant a premium price.

Background and objectives

Protected lettuce crops are vulnerable to sporadic invasions of winged aphids and moths, which colonise the plants rapidly. Customers are very sensitive to the presence of insects on the produce and their standards demand total freedom from pests. To achieve such standards, the growers are currently dependent on routine, and often intensive, applications of insecticides. The leading food retailers are urging growers to reduce their usage of insecticides but the technologies are not yet available to do so.

The commercial aim of the work was to develop an integrated pest management strategy for the control of aphids and caterpillars in protected lettuce crops. To this end, three lines of investigation were pursued:

(i)determine which were the most important species of aphids and caterpillar and when they presented the greatest threat to lettuce crops,

(ii)develop methods of reducing the pest invasion pressure,

(iii)investigate the potential of biological alternatives to chemical insecticides for aphid control.

Summary of the work and main conclusions

1.Identifying the main aphid and caterpillar species infesting lettuce crops

The first objective of this project was to determine which were the most important species of aphid and caterpillar and when they presented the greatest threat to protected lettuce crops. Such knowledge would assist in the development of pest forecasting systems and is essential in developing successful biological control measures.

Studies in the first three years of this project showed protected lettuce to be the host plant for at least four important aphid species. Of those that colonise the foliage, the currant lettuce aphid (Nasonovia ribisnigri) is specific to lettuce while the peach potato aphid (Myzus persicae), the glasshouse potato aphid (Aulocorthum solani) and the potato aphid (Macrosiphum euphorbiae) occur on a range of different plant species.

Examination of sequentially-planted crops that were not treated with insecticide showed that the populations peaked at different times of the year but the largest numbers were usually found in August and September. Monitoring of commercial crops with water traps inside the glasshouses did not reveal any consistency in population peaks of the different aphids.


Despite four years of continuous monitoring, it has not been possible to identify consistent periods of population peaks for aphids invading protected lettuce crops. It is essential therefore that protected lettuce crops receive continuous protection from foliar-feeding aphids from April through to November to satisfy the strict standards set by produce retailers.

The larvae of several moth species were also found to infest protected lettuce crops, including the silver Y (Autographa gamma), the angle shades (Phlogophora meticulosa), the tomato moth (Laconobia oleracea), the cabbage moth (Mamestra brassicae), the yellow underwing (Noctua pronuba) and at least two species of tortrix moth. Pheromone trapping studies in the early stages of this project showed the silver Y, tomato moth and tortrix moths to be the more common species and subsequent studies focused on these species. These species of moth were present from April through to October. No consistent activity patterns have been noted for any of the three main moth species over the four year period in which the trap monitoring was conducted.

Very few caterpillars were found in an unsprayed experimental lettuce crop in Yorkshire despite adult moths being recorded in traps outside the glasshouses. There were also fewer caterpillars than anticipated collected from commercial crops in other parts of the country. The latter can be explained by the inclusion of cypermethrin in the aphid spray programmes used by the growers (cypermethrin controls caterpillars in addition to aphids). More caterpillar damage was seen where cypermethrin was not used as part of the pest control programme for protected lettuce crops. Where growers waited until caterpillar damage was clearly visible on the crop before the application of control measures, inevitably there was some crop loss before control was achieved.


The Silver Y, tomato moth and totrix moths present the greatest risks to protected lettuce crops. Despite four years of continuous monitoring with pheromone traps, it has not been possible to identify any consistent activity patterns and hence lettuce crops will be at risk from April through to October. This means that continuous protection is also required against caterpillars during the risk period in order to satisfy the strict standards set by produce retailers.

2. Reducing the pest invasion pressure on protected lettuce crops

There may be three routes by which lettuce in a production house can become infested with aphids and moths / caterpillars :-

(i)Firstly, by insects surviving on lettuce debris or weeds within the glasshouse after harvest and transferring to the new crop as soon as it is planted.

(ii)Secondly, by plants becoming infested during propagation and then being transferred to the production house,

(iii)Thirdly, by winged pests flying in through the vents of the greenhouse.

Carry over from crop debris and weeds

The importance of the survival of pests on crops debris and on weeds must not be underestimated. Even a few small weeds in the least accessible places, such as the base of roof supports, may provide pests with a green bridge between crops. Furthermore, rotovating crop debris into the soil may not prevent recolonisation as aphids can work their way back to the surface. The risk can only be eliminated by the removal of all crop debris and weeds from the greenhouse immediately after harvest, using for example, a weed control spray of Gramoxone 100 (see SOLA 0225/2002) in the empty greenhouse or by burning off the crop debris with propane burners. This is a matter of good crop husbandry.


Removal of weeds and crop debris from the propagation and production glasshouses for lettuce crops is critical to the control of aphids and caterpillars to prevent a green bridge from crop to crop.

Infestation of plants in propagation

Regular examination of young unsprayed plants before they were placed in the production house confirmed that there was a potential risk of introducing aphids on propagated plants. This risk remained, though was much reduced, when the propagation glasshouse had screened ventilators. The risk of introducing pests on propagated plants that had received routine insecticide treatments was minimal.


Screening of the ventilators in the propagation glasshouse will reduce but not eliminate the risk of introducing aphids to the production greenhouse on propagated plants. This risk will be minimised also if a routine insecticide treatment is applied in propagation.

Infestation by winged pests flying in through the greenhouse vents and doors

Much of the work in this project focused on the development of an integrated control strategy based on screening glasshouse ventilators to exclude aphids and moths from the production glasshouse. For three seasons, pest establishment was monitored in unsprayed sequentially sown crops in both screened and unscreened experimental glasshouses. Where glasshouse ventilators and doors were screened with Agralan Enviromesh S48, the number of aphids on the lettuce crops were substantially reduced. Infestations that did occur in the screened house could usually be traced back to introduction on young plants or entry through damaged screens. For example, in lettuce crops that were grown sequentially from March 1999 to March 2000, there was only one unexplained record of aphids in the screened glasshouse, which was in July 1999 when five potato aphids were found on a single lettuce plant.

Screening ventilators and doors in experimental glasshouses had no apparent effect on temperature or relative humidity. There was a small effect of screening on accumulated light over the duration of each crop, which was most noticeable during the summer months. There was 2-5% reduction of accumulated light in crops between December and April, 10-11% reduction in crops between May and July, and 7% reduction in crops between August and October. There is probably less effect of shading from materials on the roof in the winter because the sun is lower in the sky and shines through the glasshouse side walls for a greater proportion of the day.

Is screening of the greenhouse vents and doors effective in providing pest control of protected lettuce crops ?

The screening studies were scaled up from small experimental glasshouses to commercial production glasshouses in 2000/2001, using Mevalon 0.6mm UV stabilised polyethylene netting on roof ventilators and PVC strip curtains on doors. The experiment compared a pest control strategy based on screening to reduce pest invasion with a routine spray programme. The studies continued to monitor effects of screening on the glasshouse environment and were extended to determine whether any loss of light affected marketable yield.

A strict and intensive pest monitoring protocol was put into practice. The crops were monitored for the presence of pests at two-weekly intervals between November and April, and at weekly intervals from May to October. On each occasion, all paths were walked and plants scanned for obvious damage symptoms (eg insect specimens, holed leaves, honeydew, etc). In addition, the crop was divided into sampling units of 30 x 13 (390) plants which were clearly bound by the glasshouse posts. Within each unit, four plants were selected at random and the presence (but not number) of aphids and caterpillars were recorded on each. This sampling process was non-destructive. Pests were identified to species level. When pests were found, an appropriate short persistence insecticide was applied to the infested crop. Monitoring then continued the following week.

See Appendix 1 for the sampling procedure used in the trials.

No aphids were collected from traps in the screened commercial glasshouse but live aphids were found on plants on five occasions between October 2000 and October 2001. On two such occasions, the plants had most probably become infested between the propagation and production glasshouses. On one occasion, small numbers of aphids were thought to have survived on debris in the soil from a previous infestation. On the other two occasions, very small numbers were found either just before or during harvest. It is not known how these aphids gained entry but no action was deemed necessary.

By contrast, aphids were collected from water traps in four of the five crops in the unscreened glasshouse. Despite the routine insecticide spray programme in this glasshouse, aphids were also found on plants on seven occasions, with most invasions occurring in late July and August.

The use of pheromone traps showed that moths were active from 1st May through to the end of October with the crops under greatest threat from mid-May to late-August. Despite this, caterpillars were very rarely found on the plants demonstrating that the protection provided by either the screens or the routine spray programme was effective.

Over the whole year, the mean number of insecticide applications was 1.0 and 2.6 per crop in the screened and unscreened glasshouses respectively. If the plants had always arrived un-infested (free from aphids and caterpillars) from propagation, then the mean number of applications in the screened glasshouse would have been reduced further. These results demonstrate that insecticide usage can be much reduced, though not eliminated, by screening glasshouses.

Screening ventilators and doors had no apparent effect on temperature or humidity in the commercial glasshouse but there was a reduction in accumulated light over the duration of each crop. The light reduction trends over the year were broadly consistent with the results from the experimental glasshouses, although the funnel shaped screens in the commercial glasshouse did cast more shade than the flat screens in the experimental glasshouse. The reduction in light did not affect the time taken for plants to reach marketable weight or the proportion of the crops that were marketable. Obviously year on year, the screens will collect dirt and therefore there will be a further potential loss of light. There was no apparent difference in disease incidence in the screened and unscreened glasshouses and the number of fungicides required was similar in both.


Screening of the vents and doorways of a commercial scale greenhouse will not prevent colonisation of lettuce crops by aphids or the intrusion of an occasional caterpillar. Screening must be supported by effective crop monitoring procedures, good cultural control practices and will require the occasional use of insecticides. Overall, this approach can reduce insecticide use by up to 75% compared to routine insecticide spray programmes.

Cost benefit analysis

The fact that pests do occasionally breach the defences in a screened glasshouse means that the pest control programme must be supported by effective monitoring procedures to determine when insecticides are required. The monitoring procedure used in this experiment was effective but may prove to be too time consuming (and therefore too expensive) for growers to adopt more generally. The time required clearly increases as the crop matures and it becomes more difficult to inspect each plant, but the average time was 2 hours per 1000m2 per sampling occasion. In this project, pest monitoring was done weekly from May to October, and at two weekly intervals from November to April. There were 33 monitoring checks over the year covering five crops and the annual cost was £528 per 1000m2 calculated using a labour rate of £8 per hour.

The cost of screening this glasshouse was £12,300 but it was a “one-off” and we anticipate that this would be reduced by approximately 50% if it became a more routine service provided by several competitive suppliers. The materials are guaranteed against UV breakdown for five years and this has been used as a conservative estimate of their life expectancy. Based on these assumptions, the capital cost (excluding interest) is estimated to be £1,230 per 1000m2 per annum. The annual bill for pest monitoring work is £528 per 1000m2, giving a total additional cost of £1,758 per 1000m2 per annum. The potential savings in terms of reduced insecticide sprays are quite small (£21 per 1000m2 per annum) because there is no reduction in labour. This is because the routine insecticide applications would normally be applied as tank mixes with fungicides and the latter will still be applied. The balance is an additional cost to the grower of £1,737 per 1000m2 per annum compared to a routine insecticide spray programme.